1. Field of the Invention
The present invention relates to the construction of an ultrasonic motor driven by the elastic vibration excited by a piezoelectric body, and to a control method for said ultrasonic motor.
2. Description of the Related Art
Ultrasonic motors that are driven by exciting elastic vibration in a vibrating body comprised of a piezoelectric ceramic or other piezoelectric body have become widely known in recent years. The operation of such an ultrasonic motor is described below with reference to FIGS. 28, 29 and 30.
FIG. 28 is a partially cut-away view of the basic configuration of a disk-shaped ultrasonic motor wherein a piezoelectric body 2 is affixed to one primary side of a elastic base 1 to form a vibrator 3. Plural projections 1a are provided on the other primary side of the elastic base 1. A moving body 4 is made by laminating a elastic body and a wear-resistant friction member together. The moving body 4 is press-contacted against the vibrator 3. An electric field is applied to the piezoelectric body 2 to induce two standing waves at a 90.degree. phase difference around the circumference of the vibrator 3, thereby exciting travelling waves of a bending vibration and driving the moving body 4 by means of friction.
FIGS. 29 and 30 show the electrode configuration of the piezoelectric body 2 in the above disk-shaped ultrasonic motor configured to excite primary bending vibration in the radial direction and tertiary bending vibration in the circumferencial direction. FIG. 29 is a plan view of side 1 of the piezoelectric body 2 in the above disk-shaped ultrasonic motor, and FIG. 30 is a plan view of side 2 of the piezoelectric body 2 shown in FIG. 28.
Referring to FIG. 30, electrodes DD and EE with a phase difference of 1/4 wavelength of the standing wave, and electrode FF equivalent to 1/2 the travelling wave wavelength, are provided on side 2 of the piezoelectric body 2. Referring to FIG. 29, electrode groups AA and BB with a phase difference of 1/4 the travelling wave wavelength, and electrode CC equivalent to 1/2 the travelling wave wavelength, are formed on side 1 of the piezoelectric body 2. Electrode group AA comprises electrode members aa1 and aa2 equivalent to 1/2 the travelling wave wavelength, and electrode member aa3 equivalent to 1/4 wavelength. Electrode group BB similarly comprises electrode members bb1 and bb2 equivalent to 1/2 the travelling wave wavelength, and electrode member bb3 equivalent to 1/4 wavelength. These electrodes are used to polarize the piezoelectric body 2, and portions of the piezoelectric body 2 corresponding to respective electrode members are polarized in the direction of the thickness thereof oppositely as indicated by the "+" and "-" signs in the figure.
The electrode groups AA and BB and electrode CC on side 1 are positioned relative to the electrodes DD, EE, and FF on side 2. Specifically, if side 1 (FIG. 29) was turned over and placed against side 2 (FIG. 30), electrode group AA would be opposite electrode DD, electrode group BB opposite electrode EE, and electrode CC opposite electrode FF.
The vibrator 3 is formed by bonding side 1 of the piezoelectric body 2 to the elastic base 1. The adhesion face between the piezoelectric body 2 and the elastic base 1 is side 1, as shown in FIG. 31 and the electrodes are flat electrodes. During use, the electrode members of each of electrode groups AA and BB are short-circuited to each other.
If voltage V1, defined by equation 1, and voltage V2, defined by equation 2, are applied from electrodes DD and EE to electrode groups AA and BB, respectively, a travelling wave, defined by equation 3, of bending vibrations is excited in the vibrator 3 from the two standing waves travelling in the circumferential direction. EQU V1=V0 sin (.omega.t) [1] EQU v2=V0 cos (.omega.t) [2]
where V0 is the maximum value of the voltage, .omega. is the angular frequency, and t is time EQU .xi.=.xi.0(cos (.omega.t) cos (kx)+sin (.omega.t) sin (kx))[3] EQU =.xi.0(cos (.omega.t-kx)
where .xi. is the amplitude of the bending vibration, .xi.0 is the maximum value of the amplitude of bending vibration, k is the frequency, .lambda. is the wavelength, and x is the position.
FIG. 31 includes a cross section of the piezoelectric body 2, and a wave diagram of the standing wave excited by one drive electrode of the piezoelectric body 2 with electrodes as shown in FIGS. 29 and 30. When electrical signals having a 90.degree. time-base phase shift are applied to electrodes DD and EE, standing wave .tau. is excited by electrode DD, standing wave .upsilon.l (solid line) is excited by electrode EE, and the moving body 4 rotates in one direction. When a signal with a -90.degree. time-base phase shift relative to the signal applied to electrode DD is applied to electrode EE, standing wave .upsilon.2 (dotted line) is excited, and the moving body 4 rotates in the opposite direction.
FIG. 32 illustrates the movement of the moving body 4. When the travelling wave is excited, an arbitrary point on the surface of the vibrator 3 moves through an elliptical path having a long axis of 2w and a short axis of 2u. The moving body 4, which is press-contacted against the vibrator 3, contacts the surface of the elastic base 1 near the peak P of this elliptical path, and friction causes the moving body 4 to move in the direction opposite the direction of wave travel at a velocity v, which is defined by equation 4. EQU v=.omega..times.u [4]
To control the rotational speed of an ultrasonic motor, it is necessary to detect the rotational speed and control the drive signal. Two methods are available for detecting the rotational speed: using an encoder or other detector, or detecting the vibrations set up in the vibrator. Because the amplitude of the vibrations set up in the vibrator is related to the rotational speed, it is possible to control the drive signal and thus control the rotational speed by detecting the vibration of the vibrator.
When a vibration detection electrode is provided on the piezoelectric body, a charge approximately proportional to the amplitude of the vibration induced in the piezoelectric body is produced at the vibration detection electrode of the piezoelectric body. It is therefore possible to detect the amplitude of the vibration set up in the vibrator by detecting the vibration-induced charge of the vibration detection electrode. In the electrode configuration shown in FIGS. 29 and 30, electrode FF is used as the vibration detection electrode. Electrode FF is sized equivalent to 1/2 the wavelength of standing wave .nu.1, and is positioned centered on the first 1/2 of the second wave of standing wave .nu.1 (FIG. 31).
A block diagram of rotational speed control in an ultrasonic motor using vibration detection as described above is shown in FIG. 33. An indicator of the magnitude of the amplitude in the vibrator is obtained by detecting the amplitude of the output signal from the vibration detection electrode FF of the piezoelectric body 2. By inputting this value to the control circuit, the drive circuit is adjusted to generate a drive wave yielding the desired rotational speed.
However, with the piezoelectric body having an electrode structure as shown in FIGS. 29 and 30, because both of the standing waves .nu.1 and .nu.2 excited by electrode DD have a high or low peak at the center of electrode FF, it is possible to detect, among the elastic travelling waves excited by the piezoelectric body, a component of standing wave .nu.1 or .nu.2 using electrode FF.
However, the components of the charges induced by the standing wave .tau. are mutually cancelled, and the components due to the standing wave .tau. among those of the elastic travelling wave induced by the piezoelectric body cannot be detected by electrode FF because the standing wave .tau. excited by electrode EE has a node at the center of electrode FF, and the amplitude of the standing wave .tau. at each end of electrode FF has the same magnitude but opposite sign.
When a change in the load that is different at electrodes DD and EE occurs, the vibration detection signal from electrode FF can respond to changes on one electrode DD side, but cannot also reflect changes on the other electrode EE side.
The effect of this change in the load is described more generally below.
Shift .delta.t to time, shift .delta.x to position, and shift m to amplitude as shown in equation 5 occur for a variety of reasons in the elastic travelling wave exciting the vibrator, and standing wave components often remain. EQU .xi.=.xi.0(cos (.omega.t) cos (kx)+m.xi.0 sin (.omega.t+.delta.t) sin (kx +.delta.x) [5]
Shift .delta.t with respect to time is often due to differences in the impedance of piezoelectric bodies and other such differences, while shift .delta.x to position is usually due to such factors as an offset in the electrode pattern. Shift m to amplitude is generally due to varieties in the composition of the piezoelectric bodies or drive circuits.
In addition, because the vibration detection electrode and drive electrode are provided on the opposite side of the piezoelectric body on which the polarization electrodes used to generate the standing waves are provided, it is difficult to precisely position the electrodes on the piezoelectric body during manufacture, and an offset easily occurs. When the vibration detection electrode is offset from the specified position, its position relative to the standing waves set up in the vibrator is also offset.
When there are residual standing wave components and the position of the vibration detection electrode is offset, the output from the vibration detection electrode changes dependent upon the direction of ultrasonic motor rotation (FIG. 34) even though the frequency of the elastic travelling wave-induced elliptical vibration of the vibrator remains the same and the rotational speed remains the same when the direction of rotation changes. In the case shown in FIG. 32, the amplitude of the vibration detection electrode output is smaller during counterclockwise rotation when compared with clockwise rotation.
FIG. 35 is a graph of the relationship between the position offset of the vibration detection electrode and the ratio between outputs from the vibration detection circuit during counterclockwise and clockwise rotation for a given time shift .delta.t. When the offset of the vibration detection electrode increases, the difference in the amplitude due to rotation direction becomes pronounced.
When the direction of rotation is reversed by inputting drive signals with the same amplitude but a phase shifted by -90.degree., the output amplitude from the vibration detection signal changes greatly.
FIG. 35 shows a graph of the relationship the rotational speed and the output amplitude from the vibration detection electrode in an ultrasonic motor having an offset vibration detection electrode. As shown therein, the rotational speed is different depending on the direction of rotation though it is proportional to the amplitude irrespectively to the direction of rotation.
In FIG. 35, the amplitude in the clockwise rotation becomes smaller than that in the counter clockwise rotation at the same rotational speed. This is due to many factors such as position offset of the vibration detection electrode to the standing wave, unevenness of the press contact of the moving body and so on, as stated in the above. Due to these reasons, the rotational speed varies greatly depending on the rotational speed in a conventional ultrasonic motor as mentioned above and it does not correspond to the amplitude one to one. Accordingly, the conventional ultrasonic motor has a disadvantage in that it is impossible to control the rotational speed using the output amplitude from the vibration detection electrode.
To avoid this, the vibration detection electrode must be precisely positioned relative to the standing wave.
Furthermore, a common potential is required to apply the drive signal to two drive electrodes on the piezoelectric body and generate a field in the thickness direction of the piezoelectric body. In general, the ground potential is used as the common potential and is connected to leads on the elastic base, which is often metal, to provide the common potential. Flexible leads are also used to connect the drive electrodes and vibration detection electrode on the piezoelectric body with an external circuitry. A common electrode electrically connected to the elastic base is also provided on the piezoelectric body to simplify connection, and the common potential is often obtained from this common electrode through the flexible leads. The leads and common electrode of the piezoelectric body are connected using the elastic base bonding the piezoelectric body or part of the circumference of the drive electrode of the piezoelectric body as the common electrode.
If when the leads are connected to the flexible base of the vibrator the connection is made at the outside circumference of the vibrator, when the vibrator is excited in an vibration mode wherein the outside circumference is the free end (See FIG. 37), the mass of the lead wires and connector parts prevents vibration and drive efficiency drops. If connection between the elastic base and lead is made using a heating process, the entire vibrator is affected by the heating, and the polarization state of the piezoelectric body is also affected.